(1) Field of the Invention
The present invention relates to a manganese-activated zinc silicate phosphor, and particularly relates to techniques for preventing the phosphor from being deteriorated by ultraviolet rays, and preventing the image quality of a gas discharge display device that uses the phosphor from being deteriorated over time.
(2) Description of the Related Art
A manganese-activated zinc silicate phosphor is commonly used as a green light emitting material for fluorescent lamps and plasma display panels (herein after called “PDPs”). The green light emitting material uses ultraviolet rays as an excitation light source. The manganese-activated zinc silicate phosphor is particularly often used for gas discharge display devices, such as the PDPs, because of its high color purity and high luminous efficiency.
The parent body of the manganese-activated zinc silicate phosphor, which is zinc silicate, is expressed by a general formula Zn2SiO4. The chemical composition of the manganese-activated zinc silicate phosphor is expressed by Zn1.9Mn0.1SiO4. However, practically, the manganese-activated zinc silicate phosphor is used with excessive amounts of silicon compared to the stoichiometric composition to gain high luminous efficiency, as described in Phosphor Handbook (hereinafter called the “Document 1”, edited by Keikotai Dogakukai (Phosphor Society), published by Ohmsha, Dec. 25, 1987, P 219-220).
Meanwhile, the manganese-activated zinc silicate phosphor is manufactured by dispensing and mixing a silicon source such as silicon dioxide, a zinc source such as zinc oxide, and a manganese source such as manganese carbonate so that the silicon component becomes excessive to some extent compared to the stoichiometric composition as described in the Document 1, and performing a heat treatment (baking process) at approximately 1200° C. in the natural atmosphere or the reduction atmosphere.
The heat treatment is performed in such a high temperature atmosphere, and therefore the zinc component is readily sublimated from the surface. As a result, compared to the chemical composition in the whole particle, the silicon becomes excessive in a region including the surface of the phosphor particle and the vicinity of the surface (This region is herein after called “the surface region”). A part of the silicon in the surface region is believed to exist as silicon dioxide.
By the way, the manganese-activated zinc silicate phosphor has a problem to be solved, that is, the luminous efficiency might be deteriorated over time, depending on the way it is used. To solve such a problem, Japanese Examined Patent Publication NO.H06-62944 (hereinafter called the “Document 2”) discloses a technique for forming a silicon nitride compound layer on the surface of the phosphor particle.
In addition to the Document 2, Japanese Laid-open Patent Application No.2002-309248 (hereinafter called the “Document 3”) tries to increase the resistance to the deterioration over time by adjusting the composition ratio in the whole phosphor particle, more specifically, the Mn/Zn atomic ratio and the Zn/Si atomic ratio in the whole particle.
The method disclosed by the Document 2 is effective in the case where an ultraviolet ray with a comparatively long wavelength generated from mercury is used, like in the case of a fluorescent lamp. However, it can not sufficiently reduce the deterioration over time in the case where ultraviolet rays having a short wavelength and high energy is used as the excitation light, like in the case of the PDPs.
The method disclosed by the Document 2 has another problem as well, that is, if a compound layer other than the manganese-activated zinc silicate phosphor is formed on the surface of the phosphor particle, the ultra violate ray, which is the excitation light, and the visible light generated by the light emission might be absorbed by the compound layer, and this greatly deteriorates the luminous efficiency.
Meanwhile, the inventors manufactured several trial models of the phosphor according to the Document 3 for each of which the composition ratio in the whole phosphor particle is modified, and the inventors found that it is difficult to reduce the deterioration over time and the phosphor of the Document 3 is not practical.
The inventors found the following facts by a keen examination.    (1) In the phosphor particle, mainly the surface region of the particle contributes to the light emission.    (2) The composition ratio in the whole phosphor particle does not necessarily equal to the composition ratio in the surface region of the particle.
In other words, the inventors found that simply adjusting the elemental composition ratio in the whole particle is not enough to control and stabilize the elemental composition ratio in the particulate surface region of the particle, which contributes to the light emission.